Jake, this may cause other problems depending on the rest of your form's design, but the easiest way I've found to prepopulate a computed rich text field from another document is to enable the "inherit values from selected document" setting. As I'm sure you know, this setting isn't limited to true response documents; if you can easily retrieve the configuration document's UNID, set the computed RTF to the name of the config doc's RTF, then append "&ParentUNID=...." to the OpenForm portion of the URL. This causes the field on the new document to default to the existing value on the configuration document, regardless of size. As I mentioned, however, this can cause other issues: namely, if you have other fields with default or computed values that reference field names, it can cause those formulas to evaluate incorrectly. But I've used this approach often with fairly consistent success.
Jake, this may cause other problems depending on the rest of your form's design, but the easiest way I've found to prepopulate a computed rich text field from another document is to enable the "inherit values from selected document" setting. As I'm sure you know, this setting isn't limited to true response documents; if you can easily retrieve the configuration document's UNID, set the computed RTF to the name of the config doc's RTF, then append "&ParentUNID=...." to the OpenForm portion of the URL. This causes the field on the new document to default to the existing value on the configuration document, regardless of size. As I mentioned, however, this can cause other issues: namely, if you have other fields with default or computed values that reference field names, it can cause those formulas to evaluate incorrectly. But I've used this approach often with fairly consistent success.